The invention relates to a method of testing a test piece, in particular in bar form, the one end of which is connected to a plate and the opposite end of which is connected to a first load head, which can be spatially adjusted as desired by a driving means. In this method, load variables applied to the load head are determined in absolute amounts, and the direction and the spatial displacement of the load head is measured in relation to a fixed plate, so that the boundary conditions of the test piece can be specifically set and varied by the displacement of the load head. By comparison of the actual values with freely programmable predetermined desired values of the boundary conditions or limiting conditions, a testpiece end value in the form of a displacement variable and/or force variable can be determined as a control variable for the driving means.
An example of this method is disclosed in German Patent Specification No. 3,425,359; the subject matter of which is incorporated by reference. The test apparatus used for the disclosed method substantially comprises a test frame with at least one abutment arrangement for supporting the forces acting on the component under test, at least one load head, which is displaceably connected to a driving means and the bearing surface of which could be rigidly connected to the component for introducing loads into the latter, as well as measuring means for determining the force variables applied by the driving means to the load head. In this method, the only way in which the boundary conditions of the component can be varied before and during the test without exchanging the load head is by the spatial displacement of the latter for any number of degrees of freedom. The force variables can be determined vectorially and for a preselected number of degrees of freedom by the measuring means. Further measuring means for determining the spatial displacement of the bearing surface with respect to the opposite abutment arrangement are built-in. For displacing the load head, a controlling means is provided, in which a control variable for the driving means can be derived in a process computer from the measured values determined by the measuring means and the preselected desired values of the boundary conditions. In this case, a longitudinally displaceable loading pendulum, which is articulated by one end to the test frame and by the other end to the load head, is provided as driving means for each degree of freedom. These loading pendulums may be designed as hydraulic piston-cylinder arrangements with admission possible at both ends. The load head itself may be designed as a plate arrangement, the loading pendulums being articulated on its edge regions. For vectorial determination of the force variables applied to the load head, two force sensors are provided, arranged axially apart, on each loading pendulum for the measurement of the normal forces and bending moments in two cross-sections of the loading pendulum. A displacement sensor is also provided.
With this known test apparatus, all of the load-dependent boundary conditions can be set or varied in a defined manner before and during the test without exchanging the load head. Something which is essential for the test apparatus is the rigid connection between the test piece and the load head, irrespective of the boundary conditions chosen. The component must therefore be firmly connected to the load head at its sectional surface. The boundary conditions of the test piece are influenced by the movement of the load head and consequently can be specifically set and varied. By comparison of the actual values and desired values of the boundary conditions, the control variable can be determined for the driving means. Consequently, even in the case of a nonlinear relationship between force variables and displacement variables, all stress resultants that are structure-mechanically compatible with the component, that is to say force variables and displacement variables and consequently all boundary conditions, can be applied by step-by-step displacement of the load head, with the result that a determination of the bearing behavior of the component in dependence on the preselected boundary conditions is possible.
Thus, with the known test apparatus, it is intended for any boundary conditions of the bar-shaped test piece to be set. Therefore, the control variables are bar end values, to be precise bar end displacement variables, bar end force variables or generally a combination of these variables. The load plate with the bar-shaped test piece connected thereto consequently has to be assumed to be rigid and actually must be sufficiently stiff up to the (theoretical) end of the bar, i.e. including the fastening construction.